1. Field of the Invention
The present invention relates to apparatus and method for forming an image on a recording medium from an electrical signal and more particularly to improvements in electronic imaging apparatus and method of the kind using an array of individually addressable light valves.
2. Brief Description of the Prior Art
U.S. Pat. No. 4,229,095, entitled "Electro-Optical Color Imaging Apparatus" and issued Oct. 21, 1980, discloses various embodiments of electronic color-imaging apparatus that utilize arrays of separately-addressable, pixel (picture element) sized, electro-optical means to effect multicolor exposure of panchromatic imaging media. One preferred kind of electro-optical means disclosed in that patent is a light valve comprising a panel of ferro-electric ceramic material, such as lanthanum doped lead zirconate titanate (PLZT) sandwiched between crossed polarizers and activated to operate in a quadratic Kerr cell mode. An array of such light valves comprises a panel of PLZT material with a plurality of interleaved electrodes formed on one major surface in a manner facilitating the selective application of discrete electrical fields across (in a direction perpendicular to the direction of viewing) discrete surface areas of the plate. Upon application of such fields, the PLZT material becomes birefringent and rotates the direction of polarization of incident light by an extent dependent on the field magnitude. This results in the transmission of light through the PLZT panel and polarizers varying as a function of the electric fields. A color image is formed electronically by selectively opening and closing such light valves in synchronization with the energization of red, green and blue exposing sources and according to the red, green and blue color information for the pixels of that image.
The above-mentioned patent also teaches that, for continuous tone imaging, the electrical energization of the light valves can be varied to provide different exposure levels and thus image density variations, i.e., a gray scale. Two different modes for varying electrical energization to change exposure (and thus density) of a pixel are described, viz: (1) varying the voltage magnitude applied during a nominal exposure period and (2) varying the periods of voltage application with a nominal voltage magnitude.
U.S. Pat. No. 4,378,568 entitled "Light Valve Imaging Apparatus and Method For Providing Gray Scale," and issued Mar. 29, 1983 in the name of J. M. Mir, discloses light valve imaging wherein pixels of the array are selectively operative for one or more of a plurality of discrete sub-period activations at constant voltage. The selected combination of sub-period activations effects proper total light transmission for the pixel. In one embodiment the sub-period activations are of different duration and binarily related so that the exposure via this mode can be termed "bit-oriented."
While each of the varying electrical energization techniques described above are useful in many applications to provide varying exposure level (i.e. gray scale) imaging, problems are presented in some applications. For example, in the variable energization techniques employing different voltage levels, inter-pixel light valve non-uniformities are experienced. (For further discussion of this phenomenon see U.S. Pat. No. 4,371,892, entitled "Light Valve Imaging With Optimized Addressing Potential(s) To Reduce Inter-Pixel Nonuniformity" and issued Feb. 1, 1983 in the name of J. M. Mir.) Constant magnitude voltage techniques (using the half-wave voltage) minimize non-uniformities, but they present other difficulties. For example, the constant-magnitude/variable-period exposure approaches, when undertaken with relatively low level, integrated-circuit-compatible voltages, present difficulties as to the accuracy of light valve switching speeds.